Bottom line. After spending an hour trying to puzzle this out, I bought NGK 7131.

My notes from Chuck Baader said that 1) For racing get a plug 1 step colder than OEM. 2) Good solution is the Autolite AR53.

The AR53 is hard to find and I’m not good at patience. I wanted something I could get either locally, or at worst, a couple days.

There’s a couple ideas as to what plug we’re supposed to be using. The original stuff isn’t available anymore. Bosch now says WR8LC+. Bimmerworld and Turner say W8LCR. The common thread is the 8 which is the # designating a cold or hot plug. And Chuck said for racing do one step cold. For Bosch, that means a lower # so 7. But how to find a plug that is the same, just colder?

The way I figured it out was to go to NGK. The Bosch WR8LC plug corresponds to the NGK ZGR5A. In the NGK world the colder plug is a higher # so 6. That led me to the ZGR6A and BPR6ES. Those don’t get a lot of hits, but if you use the NGK part # for BPR6ES…7131, now you can find them anywhere. I got mine at the local autoparts store.

I would like to understand why colder? For a turbo, when you up the boost, you are basically putting more energy into the combustion. Colder plugs allow that energy to be dissipated from the plug faster. If your plugs get too hot you risk pre-ignition, but if they are too cold you risk fouling.

With our cars, same as octane rating discussion. If you are using your engine as intended without any issue/bad fuel regulator or air leaks or made any changes this change would be unnecessary.

This is what happens when they are too hot, but I have yet to see an E30 have this issue.

Well, this isn’t an issue I know a lot about. My perception is that the hotter the engine, the colder plug you want. The colder plug runs cooler because it has less insulator area to absorb heat. So maybe there’s some preferred equilibrium temp.

I don’t agree that we’re using our engines as intended. Sure, it was designed to be able to tolerate hour after hour on the Autobahn, but even there isn’t WOT all the time. Besides, our wouldn’t have been optimized for the Autobahn, it would have been designed to perform reliably under a broad range of conditions and usage patterns. In contrast, we’re trying to optimize performance and reliability under a relatively narrow set of conditions. But that doesn’t mean I’m right. I don’t have any book-learning or experience in this.

The AR53’s have always been in stock for me at my local Advance Autoparts.

Should you be in your local autoparts store again in the next year, ask them if they still have them in stock pls. I went to an Advanced Auto last night because the Internet was telling me they had them in stock. But when I got to the place the counter guy said “no way, no one’s gonna have those. Gotta order them”.

What I meant was we have original components everywhere. And if the engine oil and water temps stay put, the interface for the combustion should not change drastically on the long term

I wouldn’t assert a drastic change, but I figure the average temp of spark plug’s environment is hotter when the average rpm is 5000 at WOT then the average environment planned for in the engineering assumptions. Also, a hot plug warms up faster. Maybe the plug warming up in a hurry has positive effects to cold engine mpg and emissions, I dunno.

That kind of conjecture makes it easy to buy into the idea that the better plug for racing is a bit colder then OEM. That is to say, handles a hot environment better.